Mesh/edit mode: how rotate TOGETHER with armature
You can modify the Armature in pose mode, then apply the armature modifier to the mesh and apply pose as rest pose
This will let you modify the bones and mesh at the same time and applying the pose bakes the changes into the natural position of the mesh and armature
You'll then have to reparent the armature but that's easy
default rest pose:

modified pose:

apply the armature:

apply the pose:

new rest pose!: